The present invention generally relates to the field of liquid chromatography. More specifically, the present invention relates to fittings used as coupling devices that couple narrow diameter tubing and Chemical Process Industry (CPI) ports of an apparatus together.
It is desirable to make connections of small diameter tubing to a receptacle of an apparatus in such a way as to allow high pressure levels of liquid to be attained in the tubing without fluid leakage at the connections. It is additionally desirable to make a union between the tubing and an apparatus in such a way as to maintain a fluid stream of a consistent diameter as the fluid is pumped under pressure to or from the apparatus through the tubing.
To those skilled in the art, a preferred method of making the connection between tubing and a fluid handling or analysis apparatus is to use a Chemical Process Industry (CPI) port, or 10-32 CPI port, or HPLC port as part of the apparatus. These terms refer to a port having a cavity that includes a threaded section, a conical section which reduces in the diameter and receives a compression ferrule, and a bottom cylindrical section. The bottom cylindrical section has a diameter that allows a snug fit of the tubing. The bottom cylindrical section has a flat bottom, against which is intended to form a void free interface between tubing and the CPI port. Together, the threaded section, conical section, bottom cylindrical section and flat bottom of the bottom cylindrical section comprise a receptacle or port into which is inserted a tube. The tube passes through a conical coupling device or ferrule that is inserted into the CPI port with the tube. The conical coupling device or ferrule is compressed against the tube as the coupling device enters the conical section of the CPI port, thereby clamping and sealing the tubing in the CPI port.
Present coupling devices for coupling tubing to the CPI port are limited with regard to the maximum operating pressure which can be attained with their use. The coupling devices are additionally limited with regard to positive means to eliminate a parasitic effect known to those skilled in the art as ‘dead volume’ of a connection. Dead volume refers to any deviation from a cylindrical fluid path through the tubing and connection into the CPI port. This effect occurs when the distal end of the tubing is not secured completely into the CPI port with maximum insertion of the tubing against the flat bottom of the bottom cylindrical section. The effect of dead volume can also be present when tubing is deformed due to over tightening of compression fittings, or improper cutting of tubing ends.
Present coupling devices consist of a threaded member which screws to the CPI port and drives a cone shaped ferrule about the tubing into the CPI port to provide clamping and sealing forces on the tubing. One present coupling device is a one-piece molded combination coupling device which combines the threaded member and the cone shaped ferrule into a single unit. The combination coupling device is convenient, but has undesirable limitations in maximum holding pressure. The combination coupling device can also create a parasitic volume at the interface between tubing and the CPI port, thus causing an undesirable dilution of chemicals being transported in solution through the tubing. Another coupling device is referred to as a ‘two piece fitting’. The two piece fitting is comprised of a cone shaped ferrule, which is compressed by a separate threaded section, to form a seal about the tubing. The two piece fitting has an advantage over the combination device because it can withstand slightly higher pressures, but is not adequately foolproof in it's ability to avoid creation of parasitic dead volumes. Yet another coupling device applies a force by means of an internal spring against the cone shaped ferrule along the axis of fluid transport within the tubing. The axial force is continuously applied as the coupling device is tightened to provide clamping and sealing force to the cone shaped ferrule, thereby reducing the possibility of a parasitic dead volume being created. This approach represents the state of the present art with respect to elimination of parasitic dead volume, and ease of use.
All three aforementioned approaches are limited by the ability of a single cone shaped ferrule to hold the tubing by means of friction applied by virtue of the compression of the cone shaped ferrule within the CPI port. Furthermore, in the case of the spring assisted coupler, if a connection is attempted while fluid is being pumped to or from the tubing, fluid pressure will build as the ferrule is being compressed. This causes the tubing to slide back against the spring force along the axis of fluid flow, causing a parasitic volume to be formed at the junction between the interface of the tubing and the CPI port. Also, in each of the previous approaches, even when the fitting is properly installed and using carefully prepared tubing, there is a negative artifact of the design of the CPI port itself. The negative artifact of the design allows radial migration of fluids at the interface between the tip of the tubing and the flat bottom of the CPI port.
Another aspect of present day coupling devices is the practice of using tubing of a smaller diameter than the bottom cylindrical section of the CPI port. The bottom cylindrical section is located at the bottom of the CPI port and is usually one sixteenth of an inch in diameter. Tubing of one thirty-second inch diameter are commonly used with special ferrules in which a cylindrical section and a cone shaped section are combined end-to-end in a unified compression fitting and diameter adaptor. The cylindrical section forms a sleeve, which adapts the diameter of the inner tubing to the diameter of the CPI port. A shortcoming of this approach is that the cone shaped section compresses, while the cylindrical shaped adaptor does not. This causes a void volume about the perimeter of the tubing and the perimeter of the cylindrical shaped section of the ferrule-adaptor. Furthermore, this type of ferrule-adaptor is supplied with a fixed length cylindrical section, hence it is unable to adapt to CPI ports with deeper than average bottom cylindrical section.
A final aspect of present day coupling devices lies in coupling device failure. When pressures of fluids which are being transported through the port increase, a component of the pressure exerts a longitudinal force along the axis of the tubing being clamped by the compression ferrule. This longitudinal force is countered solely by frictional force between the inside of the ferrule and the tubing where the ferrule contacts the tubing. As fluids within the tubing and volumes are pressurized and de-pressurized, a phenomenon known as ‘creep’ can occur. Creep causes an ever increasing void to form between the tip of the tubing and the surface of the flat bottom of the CPI port. Eventually, the tip of the tubing will be pushed back towards and through the ferrule, causing it to leak severely, incapacitating the analytical system, and possibly creating a safety hazard. To those skilled in the art, this failure is known as a ‘tubing blow-out’.
What is needed is a coupling device which can relieve the longitudinal forces between the tubing and ferrule, and allow greater longitudinal force to be applied to the tubing during and immediately after the application of radial compressive forces to the ferrule. Furthermore, it is desirable to have the ability to positively remove parasitic volumes created by either compression of the cone shaped ferrule, or variation in CPI port geometry, especially depth. Furthermore, it is desirable to have the ability to adapt narrow gauge tubing of 1/32″ and lower to be connected to CPI ports manufactured for 1/16″ tubing, without the use of a separate sleeve type adaptor. And furthermore the ability to capture or retain the tubing in the event of failure of the primary ferrule is desirable. Wherein, retaining the tubing should be accomplished in such a way as to eliminate creeping effect of dead volume, and further to eliminate ‘tubing blow out’ failure. Ideally, the aforesaid features would be implemented by a design which could be tightened by a single method of activation to couple the tubing to the CPI port. Further, it is desirable that the coupling device be compact as to allow insertion in areas congested with various components of an HPLC or UHPLC apparatus. Also, it is desirable to apply adequate longitudinal force to the tip of the tubing, thus allowing for a new type of primary seal to be achieved at the tip of the tubing and achieving a connection for communication of fluids with the lowest practical parasitic volume. That is to say, it is desirable to achieve a diskular seal at the bottom of the port, rather than a conical seal at the compressed ferrule.
It is an object of the present invention to provide a device that allows coupling between narrow tubing and a fluid handling device using a CPI port in such a way as to eliminate fluid leakage when fluid is pressurized.
It is an object of the present invention to provide a device that employs a positive method to eliminate dead volume at the interface between tubing and an interior of a CPI port.
It is an object of the present invention to provide a device to apply additional forces within an area of interface between tubing and a CPI port in such a way as to improve the ability to resist tubing creep.